SILK 'ATI-is. TTTANATES, l-TC. 



tri\ i- nearly perpendicular to 100 or the b cleavage, the cleavage 



piece yielding a nearly symmetrical interference figure. Opti- 

 cally (+). 



All)ite is associated with orthoclase ;inl microcline in the gran- 

 syenites, gneisses, and schists, and in the microcrystalline 

 ground mass of porphyries as \vell as in phenocrysts. It crystal- 

 \\itti orthodase, forming the series of alkali feldspars, and 

 with anorthite to form the soda-lime feldspars. 



In decomposing it resembles orthoclase in forming kaolin, but 

 not quite as easily, since in the same sections the albite may appear 

 clear and fresh, while t he orthoclase will be clouded by kaolinization. 

 It is associated with hornblende in diorite, and with pyroxene in 

 gal>l>ro; here the feldspars often alter to x a substance known as 

 saussurite, a mixture of albite, zoisite, etc. 



Albite may also occur as a vein mineral in phyllites and clay- 

 slates. Artificially albite is formed under the same conditions as 

 orthoclase, except that sodium salts are substituted for potassium. 



ANORTHITE 



Anorthite. Calcium aluminium orthosilicate, CaAl 2 Si 2 Og ; 

 Ca<) = 20.12, A1 2 O 3 = 36.72, SiO 2 = 43.16; Triclinic; Type, 

 (Vntrosymmetric; a : b : c = .635 : 1 : .550; a = 93 13'; p = 

 115 55'; - = 91 12'; 100 A 010=83 54'; 100A001=63 57'; 

 010*001=85 50'; Common forms, c (001), b(010), m(110), 

 M(110), x(101), y(201); Twinning, albite law common, other 

 laws less so ; Cleavage, basal perfect, 010 less so ; Brittle ; Fracture, 

 uneven ; H. = 6-6.5 ; G. = 2.74-2.76 ; Color, white, gray to reddish ; 

 Streak, white; Transparent to opaque; a = 1.574; P = 1.579; 

 V = 1.586; Y- a = .012; Optically (-); 2V = 77 18'; Bx a is 

 53 14' to the normal to 001 and 58 to the normal to 010. 



B.B. Fuses with difficulty (1532 C.) Decomposed with HC1 

 gelat ini/ing. The solution freed of silica yields tests for calcium. 



General description. Anorthite in the pure state, unmixed 

 with albite, is very rare; it has been described as occurring in the 

 lavas of Monte Somma, in rounded grains and in small complex 

 crystals, where it is associated with hornblende and biotite. It has 

 also been identified at Raymond, Maine, in a metamorphic limestone. 



Twinning is polysynthetic, after the albite law, with twinning 

 st nations on the base ; other twins are not so common. 



In sections anorthite is colorless and much like orthoclase in re- 



